The invention disclosed and claimed herein generally pertains to a mechanism of the type which employs fluid under pressure to selectively pre-load the bearings which support a machine tool spindle or like rotatable member. More particularly, the invention pertains to a mechanism of such type which allows the bearings to be axially displaced as they are pre-loaded, and at the same time prevents radial displacement of the bearings. Even more particularly, the invention pertains to a mechanism of such type which is greatly simplified over prior art devices, and which may include an improved means for sensing spindle thrust.
The importance of applying a proper amount of pre-load to the bearings which rotatably support a machine tool spindle, as the spindle drives a tool to perform a cutting operation upon a workpiece, is very well known in the machine tool arts. If a bearing pre-load is insufficient, there will be play or chatter in the spindle bearings, and the axis of the spindle will be able to deviate to some extent from the axial position required for true and accurate cutting. On the other hand, if bearing pre-load is too great during spindle operation, excessive heat may be generated, eventually causing the bearings to fail.
In certain currently available mechanisms for applying a variable pre-load to spindle bearings, the pressure of fluid in an associated fluid system is adjusted to vary the pre-load pressure on one race of the bearing. An important advantage of such mechanisms is that they enable spindle bearing pre-load to be adjusted with comparative simplicity to optimize spindle operation for different conditions. Thus, by corresponding adjustment of fluid pressure, bearing pre-load can be increased for operation of the spindle at low speed and high thrust, but can be decreased for high speed spindle operation to avoid generating excessive heat. However, present mechanisms of such type are often mechanically complex in that they require a number of pistons, plungers or other movable elements. In addition, in such mechanisms it has generally been necessary to allow a small amount of clearance between the bearings and the bore wall of the spindle housing, i.e., to provide a slip-fitting relationship between the bearings and the bore wall, as opposed to a close-fitting relationship. Such slip-fitting relationship is necessary to allow axial displacement of the bearings as fluid pressure is varied, so that the pre-load force can be transmitted to all of the bearings of the pre-load mechanism. However, if the spindle bearings are slip-fitted within the bore, the spindle supported by the bearings will be able to move radially during cutting operations, adversely affecting the accuracy or precision of cuts made upon workpieces by spindle-driven tools.
In the present invention, a mechanism is provided which has all the attendant advantages of conventional spindle bearing support mechanisms of the type which employ fluid under pressure to preload machine tool spindle bearings. The invention however, achieves a substantial simplification over various conventional mechanisms by eliminating the need for pistons or other moving parts which the conventional mechanisms tend to require. In addition, the invention is structured to radially stiffen the bearings and the spindle, to prevent deviation of the spindle axis from the axis position which is required to perform accurate cutting or machining operations upon workpieces. The invention is usefully structured to include a means for readily sensing spindle thrust during cutting operations.